1. Field of the Invention
This invention pertains generally to the field of nuclear magnetic resonance (NMR) tomography and in particular to retrospective clustering and analysis of NMR k-space measurements in the cardiac phase-respiratory phase plane. The processing system and method effect the reduction of cardio-vascular and respiratory flow motion artifacts in images and the quantitative characterization of cardiac structure and function both with and without imaging.
2. Description of the Prior Art
NMR spectroscopy, the process of analyzing a small sample in a uniform magnetic field and obtaining radio frequency data resulting from precisely pulsed radio frequency excitation, were invented by Block and Purcell. In the past 16 years, NMR analysis by spectroscopy has shifted from physical chemistry to biological chemistry and biological medical applications; i.e., biopsy samples of normal and diseased tissues. Lauterbur and Damadian and others separately invented the utilization of NMR principles to produce an image. (R. Damadian, Science 171,1151, 1971; P. C. Lauterbur, Nature 242, 190, 1973 and P. C. Lauterbur, Pure and Applied Chemistry, 40, 149, 1974). The resulting devices, NMR imaging systems, produced two dimensional and three dimensional data in which the gray scale represented is a function of a number of parameters, including for example the three parameters nuclide density, T.sub.1 (longitudinal relaxation time) and T.sub.2 (transverse relaxation time) especially in an anatomical image.
NMR imaging techniques are disclosed, for example, in "Proton NMR Tomography" P. R. Locher, Philips Technical Review, Vol. 41, 1983/84, No. 3, pages 73-88, the contents of which are incorporated herein by reference as a background of NMR imaging technology.
In vivo NMR imaging of biological tissue is rendered more difficult by movement of the biological tissue. This is especially true for example, in cardiac NMR imaging, as a result of the heartbeat. In order to prevent blurring and movement artifacts in cardiac NMR imaging, it is known to synchronize the measurements to the patients E.C.G., for example, as disclosed in U.S. Pat. No. 4,409,550, Fossel et al and U.S. Pat. No. 4,413,233, Fossel et al.
The noninvasive character of nuclear magnetic resonance (NMR) imaging and the absence of obscuration by bone structures make it a desirable technique for heart imaging. The relatively long scan times needed, however, give rise to motion artifacts. Synchronization of the imaging sequences to the heart cycle can greatly reduce these artifacts.
The capability of imaging in any phase of the cardiac cycle makes it possible to use NMR for volumetric measurements from end-systole and end-diastole images. Also, the evaluation of motion is possible by displaying images from consecutive phases in a movie loop. The use and benefits of NMR imaging for displaying the anatomy of the heart and great vessels, both untriggered and electrocardiogram (ECG) triggered, is well known. Apart from showing fine anatomical details in the heart, NMR heart imaging holds promise for tissue characterization as well, important for the detection and sizing of infarcts. Using velocity images, heart wall motion and blood flow speeds can also be quantitated with NMR imaging.
In the two-dimensional Fourier transform (2DFT) imaging method used, the NMR image is reconstructed from time signals by a complex 2DFT. For a 128.times.128 pixel image matrix, the signals are obtained in 128 consecutive imaging sequences or phases. For heart imaging, each imaging phase is triggered by a pulse derived from the R-wave of the patient's ECG. The delay of the pulse determines the imaging phase in the cardiac cycle. Each imaging phase consists of a series of radiofrequency (RF) and magnetic field gradient pulses, respectively, for evoking a signal and providing spatial information in the signal. A gradient magnetic field applied after an RF excitation pulse (90.degree. pulse) makes the proton spins at different locations in the excited slice precess at different frequencies. The spins then start to dephase with respect to the resonant frequency phase. For a given cardiac location the associated phase shift of the spins is proportional to the gradient amplitude of the magentic field and the time it is active. There is, in addition, a difference in phase shift for stationary spins and moving spins. For spins moving uniformly in the direction of the gradient, an extra phase shift occurs that is directly proportional to the velocity in the gradient direction.
The present invention pertains to an improved method and system for correlating NMR data with data from cardiac/respiratory monitors commonly used for conventional cardiac triggering and respiratory gating. Many approaches to this problem have been tried in the prior art. NMR data collection has been started at fixed delays after the ECG R-wave, with possible rejection of abnormal R--R intervals. Typically, in the known arrangements, each data collection is started at some fixed time delay after the R wave peak. In a multiple cardiac slice study, data collection from successive slices continues until close to the next R wave peak. If the heart beats regularly, each measurement will take place at the same phase in the cardiac cycle.
In the event of arrhythmia, however, deviations of the heart's position will occur for some of the data collection, thereby giving rise to blurring and movement artifacts. Such arrhythmia will further lead to a variation in the R wave repetition period TR, resulting in an increased noise level and degradation of the precision of the phase measurements, as well as degrading the general image quality of the primary images, as noted by C. Galonad, D. J. Drost, S. S. Prato and G. Wisenberg, SMRM, Vol. 2, 1985. It is well known in nuclear medicine that rejection of data collection taken during and immediately after an arrhythmia improves image quality and consistency. In the past, however, it has been difficult to effect such data rejection in a simple and efficacious manner.
Respiratory cycle timing of the NMR k-space sampling has also been utilized. However, discarding abnormal physiological cycles, or physiological cycle correlated sampling of k-space, may actually cause abnormalities to be missed, which would defeat the diagnostic purpose of an NMR scan. Because of R--R. variation, cardiac triggering is less effective at the end of a heart cycle when coronaries fill and the pre-R-wave shape of the heart may give clues to contraction abnormalities.
The prior art approaches tend to be complicated combinations of slices, phases, triggers, delays, windows and data rejection and reordering imposed on the NMR data collection, causing problems with uncertain heart phasing, limited time resolution, and nonuniform spin saturation. The present invention is designed to overcome these problems.